May I know if it is advisable to have different make and model of devices sitting on the same channel?

What are the foreseeable issues that could arise, even if these mix of devices can communicate over the same baud rate?

Many thanks!

 

It is perfectly acceptable, and quite common, to have multiple vendors' devices on the same Modbus RTU bus (all devices do need to be configured to use the same baud rate and parity settings, though). This is the whole point of a published, open protocol - so that devices from different vendors can interoperate.

 

All devices on the bus will have to have the same parity setting.

Most slave devices's RS-485 ports are not isolated and many do not have a signal ground connection terminal (terminals for driver lines only). The RS-485 signal ground is frequently the chassis ground. Differences in ground potential between units on a 485 bus can cause ground loops which can affect the signal propagation. But RS-485 isolator modules will usually solve that problem.

The master's polling rate will be limited by the slowest responding slave device.

Accumulated polling time for multiple devices adds up. Then the question is how critical is the update time for any given point?